Leaching behavior of major and trace elements from sludge deposits of a French vertical flow constructed wetland

Abstract

Surface sludge deposits were collected from a French Vertical Flow Constructed Wetland (French VFCW) sewage treatment plant. The objectives were to characterize the retention of major elements and trace metals within the sludge deposits particles under regular operating conditions, and the influence of extreme pH conditions on their potential release which may occur in situations when the plant malfunctions or after land application of the dredged sludge. A sequential extraction protocol was first used to assess the distribution of the elements within the sludge deposits. Results showed that most of Cu and Pb were associated to organic matter within the oxidizable fraction. Zn, Ni and Cd were distributed in several fractions, notably bound to Fe-Mn oxides and associated to organic matter. Cr was analyzed mostly in the residual fraction. Aliquot fractions of sludge deposits were also submitted to Acid and Base Neutralization Capacity tests (ANC-BNC) where the samples were suspended into acidic or alkaline aqueous solutions, and the solutions analyzed after 48 h contact time. Results showed a pH-dependent leaching profile for all monitored elements. The role of organic matter was observed for almost all metals. It was particularly dominant for Cu which was leached more extensively under alkaline than acidic conditions. Since Cu is not an amphoteric element, this leaching pattern was attributed to the leaching of organic matter which followed a similar pH-dependent profile than Cu. Spectrometric indices were used to characterize soluble organic compounds. Results showed that complex and humified dissolved organic compounds were mostly released under alkaline conditions.

Comparison of dried sludge and sludge ash for phosphorus recovery with acidic and alkaline leaching

“…(2006), Levlin (2005) and Kania et al . (2019). This result was not unexpected; phosphorus in the sewage sludge ash is mostly present in bonded forms with metals and can be dissolved completely in acidic conditions according to Eqs (); however, only Al‐P is directly leachable via alkaline dissolution (Eq. )…”

“…Plus, whereas Kania et al . (2019) achieved higher phosphorus dissolution under acidic condition, release efficiency was higher during alkaline leaching in the study of Kim et al . (2016) and it is proposed that, higher efficiency of alkaline condition might have been caused by the lower alkaline buffer capacity of the sludge.…”

“…In addition, Kania et al . (2019) and Kim et al . (2016) also demonstrated that the phosphorus content of the sludge deposit can be released by both acidic and alkaline conditions.…”

Residuals, sludge, and biosolids: Advancements in the field

“…The study showed those wetlands in operation one year or less had more abundant organic matter, less humified, and more biodegradable than more established wetlands (three years of operation or more) (Kania, Gautier, Imig, et al, 2019). To assess the ability of the sludge to retain specific elements including trace metals under both normal operating conditions and extreme pH conditions, samples from the wetlands noted above were examined for leaching (Kania, Gautier, Blanc, et al, 2019). The sludge layer properties, in particular permeability, were examined for their potential influence on hydraulic behavior in gravel‐based vertical flow constructed wetlands indicating increased organic matter improved permeability by three times, while total solids increase reduced permeability; improvements in mineralization related to drying kinetics was also described (Khomenko, Dotro, Jefferson, Coulon, & Bajon Fernandez, 2019).…”

ANC–BNC Titrations and Geochemical Modeling for Characterizing Calcareous and Siliceous Mining Waste

“…Then, pH and element release are monitored and reported as a function of the amount of acidity or alkalinity. Such experiments were used to assess element release from several types of cement and concrete matrices [20,27] into contaminated soils [28], including even natural soils [29]. Several protocols were developed for these titration experiments [30], but the main principles remain the same.…”